The present invention relates to devices for monitoring the carbon dioxide content in air being exhaled by a medical patient, and more particularly to techniques for calibrating such monitoring devices.
Medical patient connected to a respirator typically have the carbon dioxide (CO.sub.2) content of their exhaled air monitored. The carbon dioxide content was conventionally measured as a partial pressure, expressed in terms of millimeters of mercury. Heretofore, devices that performed such monitoring are relatively large, table model units which were not easily transported and which were used in the relatively controlled environment of a hospital.
Recently, a hand-held carbon dioxide monitor has been developed which offers several advantages over previous table model units. The hand-held device can easily be carried from patient to patient in an intensive care unit so that a single device can be connected alternately to the respirators for a number of patients. Thus, a single unit can be used to monitor the amount of carbon dioxide exhaled by several patients as opposed to requiring a separate table model unit for each patient. In addition, the hand-held unit can be transported in an ambulance and used by paramedics following resuscitation of accident victims.
Previous carbon dioxide monitors had to be calibrated periodically to compensate for changes in the performance of components with age and temperature fluctuation. Typically the carbon dioxide monitor executed a calibration procedure when power was first applied. In addition, during prolonged continuous use of the monitor, an internal timer periodically triggered recalibration.
During the development of a hand-held carbon dioxide monitor, it was discovered that such devices were subjected to mechanical shock during transportation. In addition, the use in ambulances subjected the monitor to rapid changes in temperature. The affects of such mechanical shock and temperature variation during periods of non-use, such as when the device is being transported to a patient, can easily be compensated for by executing the calibration routine during initial power-up of the monitor. However, the use of monitors connected to accident victims also subject the monitor to mechanical shock and temperature variation as the patient is transported from the accident scene to an ambulance and then from the ambulance to a hospital emergency room. Therefore, the accuracy of the hand-held carbon dioxide monitor drift during even short periods of operation.